This invention relates generally to computed tomography (CT) imaging and more particularly, to automatically adjusting x-ray source current to reduce image noise in a CT system.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Detector element of the array produce a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array arc rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Certain scanning parameters, such as scan rotation speed, image slice thickness, scan mode, x-ray collimation, filtration, and table speed are known to affect required x-ray source current ("mA"), which relates directly to image noise. In order to optimize image noise, for example, a faster rotation typically requires a higher x-ray tube current level. Conversely, slower rotation typically requires a lower x-ray source current level. Similarly, a thinner image typically requires a higher x-ray source current level as a compared to a thicker image.
To optimize image noise, known CT systems require an operator to consider each operating parameter to determine the appropriate x-ray source current. Specifically, in determining the x-ray tube current, the operator must consider each of the operating parameters as well as the interrelationship of each parameter. The possibilities created by the interrelationships may lead to operator confusion causing the operator to incorrectly determine the x-ray source current. As a result, either image quality is reduced or the patient may be exposed to increased x-ray dosages as a result of the incorrect x-ray current.
Accordingly, it would be desirable to provide an algorithm to facilitate optimizing image noise based upon the operating parameters of the imaging system. It also would be desirable for such algorithm to facilitate reducing x-ray dosage by matching image quality requirements and x-ray source current.